Alright, I did a little legwork today. I went to
Gregg Distributors. I think they're similar to Acklands/Grainger?
They measured a featherweight belt - 1/4" wide and 17 1/4" in circumference. They think a 2L180 Goodyear belt will fit. They didn't have any in stock but can order them for me - prepaid because it's special order.
My next goal is to try to find one elsewhere where I can make sure it's the right size - then go back to Gregg to order "in bulk". I've been told to ask for "2L V groove belts" and they come in smaller sizes though are likely not regular stock items for most industrial shops. Worth asking though.